A motor vehicle body typically includes a pair of longitudinal "rocker" panels where the vertical sides of the body intersect the horizontal floor of the body. For aesthetic purposes, the rocker panels are commonly covered or shrouded by a pair of trim panel assemblies each including a molded plastic trim panel having a facia portion shaped to fit over a vertical side of the rocker panel and a skirt portion shaped to fit over a horizontal bottom of the rocker panel. The trim panel is molded in a selected color with a plurality of integral L-shaped supports on an inside surface of the facia portion each including a platform facing the vertical side of the rocker panel and a strut perpendicular to the platform at an outboard edge of the platform. A push-in fastener of the trim panel assembly is mounted in a slot in the platform of each of the L-shaped supports. Force applied to an outer surface of the facia portion at each of the L-shaped supports thrusts the push-in fastener thereon into a socket in the vertical side of the rocker panel to secure the facia portion of the trim panel to the rocker panel without the push-in fasteners being externally visible. The strut of each of the L-shaped supports transfers or reacts to the facia portion a fraction of the thrust applied to the corresponding push-in fastener to prevent flexure of the platform of the L-shaped support. In order to mold the trim panel with such integral L-shaped supports, a relatively complex mold is required including a plurality of sliding cores which must be withdrawn from inside of the L-shaped supports in order for the trim panel to be extracted from the mold. Such sliding cores increase the cost of the molds and are responsible for visible undulations in the outer surface of the facia portion of the trim panel attributable to relatively more rapid curing of the plastic melt which contacts the sliding cores. A trim panel assembly and method according to this invention are improvements over trim assemblies having such molded plastic trim panels with integral L-shaped supports and methods of making the same.